Does turning a list into a set, then back again, cause problems in Python?

Question

I'm turning a list into a set in Python, like so:

request.session['vote_set'] = set(request.session['vote_set'])

So I can easily do a if x in set lookup and eliminate duplicates. Then, when I'm done, I reconvert it:

request.session['vote_set'] = list(request.session['vote_set'])

Is there a better way to do this? Am I potentially doing something dangerous (or stupid)?

score 5 · Accepted Answer · 2010-02-27T01:42:36.817

5

You'll lose duplicates if you actually wanted them. If this is actually a list of "votes" as your naming suggests, you'd 'lose' some :)

why not just:

if x in set(request.session['vote_set'])

if you're worried.

Although I have to wonder if that would be slower than just plain:

if x in request.session['vote_set']

And ordering, as others have mentioned, would potentially (most likely) be lost.

edited Feb 27 '10 at 01:42

answered Feb 27 '10 at 01:37

Converting to a set (which involves traversing the whole list once) just to test membership once will be slower than testing for membership in the list (which involves traversing the list until a match is found). If the set is large and you can use it more than once, at some point it will become faster; that's not likely to be the case here though. – Peter Hansen Feb 27 '10 at 15:52

score 1 · Answer 2 · answered Feb 27 '10 at 01:31

1

You'll lose ordering, if that's important to you.

answered Feb 27 '10 at 01:31

Justin R.

score 0 · Answer 3 · edited May 23 '17 at 12:13

0

This is how you remove duplicates AND maintain the order (if you care): Algorithm - How to delete duplicate elements in a list efficiently?

The other answers showed how to turn a list into a set.

edited May 23 '17 at 12:13

Community

answered Feb 27 '10 at 01:38

Hamish Grubijan

3 Answers3